Okay so what I remember is Sesshoumaru ask Kagome without her realizing it to bear him a heir. He ends up traveling with them in order to do so. A few things I remember get tells her to never bear her neck and that bathing is for mates. By about the 15 chapter he accepts he cares and then allows her to bath him. I thought I found it but it wasn't the right story. I k ow the summary says InuYasha was away while she agrees.

could it be this ?
url: inu.adultfanfiction.net/story.php?no=600005601
title: Unexpected Endeavors
Author: scottishfae

EDIT BY WICCAN: This story can be found here on Dokuga, complete in 38 chapters: http://www.dokuga.com/fanfiction/story/6046/1. This is rated MA, NO YIMs!!

So i think Kino may be describing two different fics. Part of the description definitely sounds like Unexpected Endeavors, however the bit about him traveling with them and rules about her behavior just dont match up as Sesshoumaru is actually quite affectionate, if clueless about his actual feelings, throughout the entire fic. So maybe they remember a little more of someone else has another fic in mind???

Every single one of these suggestions is MA

Honestly I'm coming up with a few...I want to say it was: A Blue Ribbon by StellaMira because I thought it started out as a fwb type of thing.

dokuga.com/fanfiction/story/7896/1?author=11361

Was it a one off? Try Fanfic He Must Be Breeding by ChaoticReverie

m.fanfiction.net/s/6742757/1/

Also try A Little Faith by Elle6778

m.fanfiction.net/s/3530724/1/

Just another lemon by Tiegrsi

dokuga.com/spark/story/6398/1

Numbing the pain by AngelicMemories

dokuga.com/fanfiction/story/1802/1?author=279

Clash of Alphas by Sarhea

dokuga.com/fanfiction/story/1802/1?author=279

Check through these